Glaucoma is a disease of the eye which is not curable, but treatable, especially when detected in an early stage. Glaucoma results in elevated intraocular pressure, or IOP which causes damage to the optic nerve and loss of vision.
The usual test for glaucoma is to measure the IOP of a patient's eye, however it has been found that the IOP is not a constant. Even in normal eyes varies throughout the course of the day. For normal eyes, the diurnal variation may range between 3 and 12 mmHg. For patients suffering from glaucoma, the diurnal variation is substantially greater, typically ranging from 11 to 19 mmHg. but may, on occasion, reach 50 mmHg. As a result, a ophthalmologist can not rely upon results from a single tonometer test given to a patient in the office to determine whether the IOP of his or her eye is elevated. When a patient is suspected of having glaucoma, it is therefore desirable to test the IOP of a patient's eyes several times during the course of a day, and over a period of several days to determine an average IOP, and the maximum IOP for the patient. To undertake such tests, it is desirable to have a tonometer which can be operated at home by the patient without the assistance of a technician.
Efforts have been made to develop a patient operated tonometer, however certain problems have been encountered in the development of the device.
One problem arises from the delicacy of the human eye. To read the IOP of a patient's eye, a probe must first be positioned against the patient's eye, and thereafter the pressure of the probe against the eye must be gradually increased until the cornea of the eye is caused to applanate. The device must read the pressure at which the cornea of the eye is caused to applanate. To measure the small pressures within the eye, pressurized air is used to telescope a probe against the eye of the patient and the pressure of the air measured to determine IOP. Even though it is customary to anesthetize a patient's eye, patients have difficulty tolerating the contact of a probe to the eye, and the patient's sensitivity has resulted in inaccurate readings of such tonometers.
Previously developed patient operated tonometers have a probe slidable within a fixed barrel, and pressurized air is applied within the barrel to move the probe against a patient's eye. Unfortunately, existing devices do not monitor the movement of the probe within the barrel, and the probes of existing tonometers can be accelerated down the barrel by the increasing air pressure applied in the barrel, such that the probe can cause injury to the patient's eye. This danger is compounded if the probe should "stick" as the pressure in the barrel is increased. When the air pressure is sufficiently high to free the probe, it may be rapidly propelled into the patient's eye causing injury.
Another difficulty encountered with patient operated tonometers is that the accurate readings of IOP requires that the measurement be taken from the center of the cornea of the eye. Existing patient operated tonometers provide for a conventional light source aligned with the center of the axis of the probe, and the patient is instructed to focus his or her eye on the light source. Such methods have not resulted in the accurate alignment of the eye because such light sources direct a wide pattern of light against the retina of the patient's eye, and the patient has difficulty focusing on the center of the source, which is required if the cornea of the eye is to be centered over the probe.
In order to provide a tonometer which can be used in a patient's home, without the existence of a technician and which will provide accurate readings of IOP, the foregoing problems must be overcome.